Some gas turbine engines are controlled by two fuel control systems, each responding to one or more sensors that provide signals indicating engine operating conditions, such as high rotor speed N2. Each control is sometimes called a "channel". Each control has a sensor to sense the engine operating condition. Ideally, the sensors are closely matched in their respective output signal levels. While the outputs do not have to be exactly the same, if one sensor is "out of the range", the other controller will try to compensate. Internal, diagnostic routines may cause the first channel to shut-down. This takes place more often when the internal filters in each controller-used to smooth response to transients-are proportional-integral because the difference between the sensor outputs increases with time, producing errors so great that the two channels would diverge, one being full on the other full off.